1. Field of the Invention
The present invention relates to an orthopedic joint prosthesis such as a hip stem, knee component, shoulder component, intramedullary nail or the like and more particularly relates to an improved taper lock connection apparatus for joining two prosthetic parts (such as for example the head and stem of a hip implant) wherein stress is reduced by removing material from either the end or the internal circumferential plane of the male taper component, thereby making the male taper component more flexible without an appreciable reduction in structural integrity.
2. General Background
It is known in the art to form a connection between two orthopedic prosthetic components using a morse taper or taper lock connection. One of the most common examples of such a connection is the neck of a hip stem that forms a connection with a correspondingly shaped socket in the ball of the femoral head component typically called the femoral head.
A hip stem neck is often frustroconically shaped and the femoral head of the prosthesis provides a corresponding frustroconical socket for receiving the neck of the hip stem. An example of such a frustroconical neck and its frustroconical socket for a hip stem and ball can be seen in the Fallin U.S. Pat. No. 5,108,452 entitled "Modular Hip Prosthesis" issued Apr. 28, 1992, incorporated herein by reference.
Morse taper or taper lock constructs have also been used for connecting knee components, shoulder components, and intramedullary nail members.
An example of an intramedullary nail with a conical male portion and an axially threaded bore formed in the proximal end of the base portion as seen in U.S. Pat. No. 5,122,141.
Extraction holes are sometimes used in taper designs to provide an attachment point for surgical removal of an implant. This feature can be seen for example in the Genesis Knee System manufactured and sold by Smith and Nephew Richards of Memphis, Tenn., assignee of the present application. In the Genesis Knee System, and attachment point for surgical removal of the implant is provided on male tapers of the tibial stem extension. An extraction hole typically has a thread pattern in the hole to accommodate a male threaded hammer to assist in surgical removal.
Tapered sleeves have been used for attachment to the frustroconical neck of a hip implant. These tapered sleeves provide a threaded hole to facilitate sleeve removal. Such tapered sleeves have been sold for years by Smith & Nephew Richards, assignee of the present application.
Many issued patents show taper lock or wedge lock connections between orthopedic implants. A recent patent is U.S. Pat. No. 5,246,462 issued to K. Bekki et al. and entitled "Insert for Identifying and Implantable Ceramic Joint Head". An object of this patent is to provide an implantable ceramic head which can be readily marked without a reduction in the strength of the ceramic. In the artificial ceramic joint head of the '462 patent, a recess is provided into which the stem is inserted. An insert with a marking is provided on the bottom of the recess. The connection between the stem and joint head is a male conical taper on the femoral stem and a correspondingly shaped socket on the femoral head.
U.S. Pat. No. 5,181,929 entitled "Femoral Prosthesis Employing a Small Ceramic Ball" discloses a metal rod having a male frustroconical end portion and a femoral head, or ceramic ball having a frustroconical blind recess, assembled by means of a conical sleeve fitment. The ceramic ball is formed of a material having an improved flexion breakage modulus and an improved elasticity modulus.
U.S. Pat. No. 5,122,141 issued to Simpson et al. discloses an intramedullary nail system and method for providing the capability of creating intramedullary nails of any desired length by a combination of a small number of base nail members adapted to be joined at any one of a variety of hollow extension nail members. Any selected extension nail member in order to prevent axial separation of the members. Additionally, each extension nail member is provided with transverse openings adapted to receive a bone screw to secure the intramedullary nail member is infinitely rotationally adjustable about fixation of the extension member with any desired degree of anteversion prior to final assembly of the base nail member with the extension nail member.
U.S. Pat. No. 4,964,869 discloses an assembly of a ceramic head fixed to a rod which assembly is a component of hip prosthesis that has improved distribution of stress transferred from the rod to the ceramic head. The ceramic head includes a blind recess, generally in the shaped of a truncated cone, and to which the male end of the rod is nested, in which this nesting together is provided without play between the ceramic head and the end of the rod in only a portion of the depth of the recess, while in the remaining portion of the recess there is no contact between the ceramic head and the end of the rod.
The Averill et al. U.S. Pat. No. 4,921,500 discloses an adaptor that is interposed between a socket and a ceramic femoral head component and a post on a femoral stem component in the femoral stem of a prosthetic hip joint to enable use of a taper within the socket which is steeper than the taper on the post as to reduce hoop stresses in the ceramic femoral head component while at the same time as accomplishing securement of the ceramic femoral head component upon the post of the existing femoral stem component.
The metal male joint part of a hip prosthesis has a taper which is less than that of the cavity of an oxide ceramic female joint part in U.S. Pat. U.S. 4,058,856 entitled "Joint Endoprosthesis". In addition, the surface of the male part has a resistance of defamation which is less than that of the core male part. The cone angle of the female cavity and the cone angle of the male joint part differ.
U.S. Pat. No. 5,080,685 discloses a hip prosthesis that uses a wedge lock taper between the femoral head and the hip stem neck or trunion.
U.S. Pat. No. 4,268,919 discloses a joint endoprosthesis that includes a pivot body with a radially extending hole, a pin press fit in the hole, a recess extending into the pin from its end of the hole, wedging means for spreading the pin from its end in the hole, wedging means for spreading the pin end in the hole in the body to securely hold the pin. U.S. Pat. No. 4,198,711 entitled "Joint Endoprosthesis" discloses a prosthesis that includes a pivot body with a radially extending hole, a pin press fit in the hole, a recess extending into the pin and its end in the hole wedging means for spreading the pin from its end in the hole, wedging means for spreading the pin end in the hole in the body to securely hold the pin.
Zeibig et al. U.S. Pat. No. 4,170,794 discloses a bone joint endoprosthesis that comprises a spherical body of the joint with a hole in it for receiving a shaft spigot, a shaft is embedded in the bone, the shaft has a spigot which is received in the hole, a spreader, comprised of a material that swells when it is wetted, is rapped over the spigot in the hole or is inside the spigot and the spreader expands radially in the hole or inside the spigot to secure the joint, shaping and profiling of the spigot in the wall of the hole for effective retention of the spreader are disclosed.
An artificial head assembly for a human femur is disclosed is U.S. Pat. No. 4,012,795 issued to Doore et al. The apparatus consists of a head portion proper and a metal pin. The head portion is a centered ceramic ball having a polished contact face of convex, spherical curvature and a blind bore tapering conically inward toward the contact face. The pin is shaped for insertion of one end into the hollow stump of the femur, the other end tapering conically at the same apex angle as the bore in the ball. A knurling of the conical face of the pin reduces compressive strength of the pin surface to less than tensile strength of the ball so that the assembled head and pin can be sterilized without cracking the ceramic head portion because of difference in coefficients of thermal expansion between the metal and the ceramic material.